A system in use today for determining an unknown position of a mobile radio receiver may utilize the satellite-based GPS system (“Global Positioning System”), and in the near future, the European Galileo system. The terms “GPS” and “Galileo” will be used interchangeably herein. Both systems work similarly, each employing about 24 to 30 orbiting satellites and each with accurately known position and time, which transmit a signal with a time stamp that indicates when the signal was sent from the satellite. In order to be able to compute the position of the mobile receiver, satellite clocks in each system are accurately synchronized to a common time reference. The mobile receiver calculates its position by the following (simplified) steps: First the time stamp of at least four satellites is extracted from received signals by the mobile receiver, and the time of arrival of the time-stamped message is recorded. The distance from each satellite to the mobile receiver is calculated by comparing the respective time stamp with its arrival time, using the accurately known propagation velocity of the radio signal. The position of each satellite can be accurately determined at any instant in time using satellite ephemeris data, which is regularly updated. Thus, the distance to at least three of the satellites can be calculated by the mobile receiver position using triangulation. However, the clock in the mobile receiver may not be fully synchronized with the synchronized clocks in the satellites. A time-stamped signal received from a fourth satellite by the mobile receiver is generally used to compensate clock uncertainty in the mobile receiver. To improve accuracy even further in determining the location of the mobile receiver, perturbation effects operative on the received signal such as atmospheric effects, earth rotation, relativity, etc., are typically included in the computation of the mobile receiver location.
The time stamp sent out from the satellites uses a spread-spectrum code with a bit rate of about 1 Mchip/s (1.023 Mchip/s). Since the speed of light (in a vacuum) is about 299,792,458 m/s, the “length” of one chip is about 300 m. This means that in order to obtain good accuracy in the position computation, the arrival time must be determined by the mobile receiver within a reasonably small fraction of a chip. Existing products are able to determine position with accuracy better than 10 m when there is sufficiently high signal to noise ratio, confirming that it is possible to determine the time of arrival of the received signal with high accuracy.
Satellite navigation applications require Galileo/GPS receivers with high positioning accuracy at low signal-to-noise ratio. This is particularly the case for applications in deep urban and indoor environments, where envelopes of buildings and vehicles attenuate the signals transmitted by the satellites, which are already very weak at about −160 dBW when unobstructively received on the earth's surface. Positioning accuracy produced by a Galileo/GPS receiver generally depends on the signal-to-noise ratio of the received signal, which is typically low in deep urban and indoor environments. The result is a degrading of positioning accuracy in view of user expectations or system requirements.
Thus, the design of an improved Galileo/GPS receiver that provides improved positioning accuracy, particularly in an environment of low received signal-to-noise ratio, would address an unanswered application need.